在使用kubectl apply操作workload产生的非预期行为

简述

• kubectl apply 对于之前本身就是apply或者create --save-config产生的resources，可能存在非预期行为(kubectl.kubernetes.io/last-applied-configuration)，例如修改原有的workload probe settings(bug at 1.18.x), 或者修改hostAlias(修改原有的hostalias的ip)

hostalias复现:

先创建一个普通的deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    sit.k8s.io/app: yaml-test
  name: yaml-test
spec:
  replicas: 1
  selector:
    matchLabels:
      sit.k8s.io/app: yaml-test
  strategy:
    rollingUpdate:
      maxSurge: 25%
      maxUnavailable: 25%
    type: RollingUpdate
  template:
    metadata:
      labels:
        sit.k8s.io/app: yaml-test
        sit.k8s.io/app: yaml-test
      name: yaml-test
    spec:
      hostAliases:
      - hostnames:
        - testaaaa.com
        - testbbb.com
        ip: 1.1.1.7
      containers:
      - image: 192.168.150.181/test/nginx-2048:latest
        imagePullPolicy: IfNotPresent
        name: yaml-test
        readinessProbe:
            httpGet:
              path: /
              port: 80
              scheme: HTTP
            initialDelaySeconds: 10
            timeoutSeconds: 1
            periodSeconds: 10
            successThreshold: 1
            failureThreshold: 3
        resources:
          limits:
            cpu: "1"
            memory: "64Mi"
          requests:
            cpu: 64m
            memory: "64Mi"
      dnsPolicy: ClusterFirst
      restartPolicy: Always

然后修改一下hostAliases的值，执行kubectl apply -f xxx.yaml --dry-run=server -oyaml

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    sit.k8s.io/app: yaml-test
  name: yaml-test
spec:
  replicas: 1
  selector:
    matchLabels:
      sit.k8s.io/app: yaml-test
  strategy:
    rollingUpdate:
      maxSurge: 25%
      maxUnavailable: 25%
    type: RollingUpdate
  template:
    metadata:
      labels:
        sit.k8s.io/app: yaml-test
        sit.k8s.io/app: yaml-test
      name: yaml-test
    spec:
      hostAliases:
      - hostnames:
        - testaaaa.com
        - testbbb.com
        ip: 1.1.1.8
      containers:
      - image: 192.168.150.181/test/nginx-2048:latest
        imagePullPolicy: IfNotPresent
        name: yaml-test
        readinessProbe:
            httpGet:
              path: /
              port: 80
              scheme: HTTP
            initialDelaySeconds: 10
            timeoutSeconds: 1
            periodSeconds: 10
            successThreshold: 1
            failureThreshold: 3
        resources:
          limits:
            cpu: "1"
            memory: "64Mi"
          requests:
            cpu: 64m
            memory: "64Mi"
      dnsPolicy: ClusterFirst
      restartPolicy: Always

会发现apply以后的结果hostAliases字段非预期（1.1.1.8直接被追加了进去，而不是替换）

或者一开始使用apply创建资源，然后删除kubectl.kubernetes.io/last-applied-configuration: ，再修改ip，再apply

如果一开始是用kubectl apply -f xxxx创建资源，然后用apply -f更新资源，则不会复现

这个也是跟k8s apply的实现方式有关系

How apply calculates differences and merges changes

Related article: K8s的一些小坑或者bug简要记录